Lambert-Eaton myasthenic syndrome (LEMS) is a rare, antibody-mediated autoimmune disorder. It can occur sporadically or as a paraneoplastic syndrome, most often associated with small cell carcinoma of the lung. The clinical presentation may be mistaken for myasthenia gravis, because the pathophysiologies of the two disorders have some similarities (Pascuzzi, 2002). Antibodies are directed against the voltage-gated calcium channels at the neuromuscular junction. The neuromuscular junction, or synapse, is the area where the nerve cells and the muscle fibers meet. Nerve cells regulate the function of the muscle fibers by controlling the frequency of the electrical signal, or action potentials, produced in the muscle fibers, resulting in a series of chemical events. These chemical events lead to muscle movement or contraction. At the neuromuscular junction, the nerve cell is known as the presynaptic terminal or membrane, where the action potential is initiated or produced, and the muscle fiber is known as the postsynaptic terminal or membrane, which receives the action potential.
Acetylcholine, a neurotransmitter, is released from the voltage-gated calcium channel of the presynaptic nerve at the neuromuscular junction. A neurotransmitter is a substance released from a presynaptic membrane that stimulates the action potential in the postsynaptic membrane. In LEMS the body produces antibodies that attack the presynaptic P/Q-type voltage-gated calcium channel (VGCC) of the neuromuscular junction. The antibodies bind to the particles that form the VGCC, causing clumping, which reduces the amount of calcium allowed to pass through the channels and also reduces the number of acetylcholine particles to be released. This attack on the VGCC, ultimately causes a decrease in acetylcholine. Anything that affects the production, release, and degradation of acetylcholine or its ability to bind to its receptor molecules on the postsynaptic membrane also affects the transmission of action potentials across the neuromuscular junction. This interferes with nerve impulses to the muscles. If the nerve impulses cannot get through to the muscle, the end result is proximal muscle weakness, hyporeflexia, and autonomic dysfunction.
EPIDEMIOLOGY AND ETIOLOGY
The incidence of LEMS is unknown, but 50% to 60% of these patients are diagnosed with non-small cell lung cancer. About 3% to 6% of patients with small cell lung cancer (SCLC) are diagnosed with LEMS. The median age of onset for LEMS is 60 years, although the disorder can occur in individuals anywhere from 7 to 80 years (Wirtz et al., 2002). If cancer has not been found in a patient presenting with LEMS, the patient should be screened for SCLC every 6 months, with chest imaging, for at least 2 years. In addition, evaluation for other autoimmune disorders, such as systemic lupus erythematosus and antibody-positive myasthenic gravis, should be considered (Pascuzzi, 2002).
If a patient is diagnosed with LEMS and a tumor is not found within the first 2 years after the onset of symptoms, cancer is unlikely; however, a latency period of 3 to 8 years has been reported (Scully et al., 1994).
RISK PROFILE
• Cancers: SCLC; breast, prostate, and laryngeal carcinomas; Non-Hodgkin’s lymphoma; and leukemia.
• Paraneoplastic syndrome in middle-aged and elderly individuals.
• Autoimmune disorders: Systemic lupus erythematosus, thyroiditis, pernicious anemia, antibody-positive myasthenia gravis, rheumatoid arthritis, vitiligo, celiac disease, type 1 diabetes, and multiple sclerosis.
• Lifestyle: Smoking (which can lead to lung cancer).
PROGNOSIS
The survival rate depends on the underlying cancer and its response to treatment. LEMS can be resolved if the tumor successfully responds to the treatment. Little information is available on the prognosis for patients with LEMS who do not have a malignancy.
PROFESSIONAL ASSESSMENT CRITERIA (PAC)
1. Slowly progressive proximal leg and arm weakness, followed by generalized weakness, which is worse in the morning and tends to improve throughout the day. Difficulty climbing stairs or rising from a chair is noted.
2. Generalized fatigue after protracted exercise, with an increase in strength after mild exercise.
3. Waddling gait and impaired balance.
4. Dry mouth, related to the effects on the autonomic nervous system.
5. Difficulty chewing and swallowing.
6. Slurred speech and jaw and facial muscle weakness.
7. Constipation.
8. Vision changes: Blurred vision or diplopia.
9. Respirations: Decreased in rate and depth if the muscles of ventilation are affected. Cough associated with lung cancer or infection may be present.
10. Deep tendon reflexes may be decreased or absent.
11. Extremities: Numbness or paresthesias.
12. Impotence in males and postural hypotension.
13. Vaginal dryness.
14. Prolonged paralysis after the use of neuromuscular blocking agents during surgery.
15. Increased weakness after administration of aminoglycoside or fluoroquinolone antibiotics, magnesium channel blockers, and iodinated intravenous contrast agents.
16. Laboratory values:
• Antibody assay: VGCC antibodies are found in 75% to 100% of patients with LEMS who have SCLC and in 50% to 90% of patients with LEMS alone.
17. CT scanning or MRI of the chest for SCLC.
19. Repetitive nerve stimulation studies: These studies confirm the LEMS diagnosis by demonstrating characteristic findings on electrodiagnostic studies.
20. Needle electromyography.
21. Single-fiber electromyography.
22. Bronchoscopy to detect SCLC.
23. Pain related to muscle stiffness.
24. Pulse oxygen for O2 saturation related to SCLS.
25. ABGs: Acidosis, hypercapnia, hypoxia.
NURSING CARE AND TREATMENT
1. Asses ventilation status: Respiratory rate and depth may be decreased, and dyspnea or cough may be present. Administer oxygen therapy. Follow aspiration precautions. Respiratory failure is rarely the presenting symptom in LEMS patients, although it can develop later in the disease process (Eaton & Lambert, 1957). Administer decongestant PRN: guaifenesin or antitussive PRN: hydrocodone bitartrate (Hycodan/Robidone). Heliox treatment is a combination of helium and oxygen. Helium is a smaller, lighter molecule, which allows for oxygen delivery through smaller airways; this treatment works best for narrowed airways.
2. Assess mobility and balance: Provide safety measures to prevent falls.
3. Assess and document muscular strength.
4. Assess and document neurologic status.
5. Provide bowel function treatment for constipation.
6. Obtain and assess laboratory test and biopsy results.
7. Medications that may be ordered:
• Pyridostigmine 30-60 mg PO q4-6h.
• Guanidine hydrochloride 5-10 mg/kg/day PO to a maximum dose of 30 mg/kg/day in divided doses.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
